I see people using these in the grocery stores. Would they need service through a cell plan in order to use them there or do most grocery stores now have wi-fi that the public can access?

I want to buy a tablet and we have DSL in the house, but I won't be able to pay for a monthly fee as well just to use it in places that don't have the free wi-fi. It would be great to find out that most grocery stores have free access, because I'm sure I would use one when shopping.